This invention relates to test devices, generally referred to as phantoms. More specifically, this invention relates to a phantom useful with nuclear magnetic resonance (NMR) scanner apparatus to carry out performance and calibration measurements.
A phantom generally comprises a test object constructed to simulate structures and conditions encountered in actual clinical use of the NMR scanner. In the case of medical diagnostic applications, the phantom can be made to simulate various types of tissue and can be used as a substitute test object in operator training, as well as a calibration device to ascertain the level of scanner performance. In some cases, it is desirable to ascertain the degree of scanner operability by frequent calibration procedures. The phantom therefore must be constructed to allow evaluation of multiple system quality parameters with relative ease and a minimum expenditure of operator time and effort.
As is now well known, NMR techniques have been developed capable of acquiring spectroscopic and imaging information about the internal anatomical features of humans, for example. This information can be analyzed to determine such tissue-related parameters as nuclear spin distribution, spin lattice (T.sub.1), and/or spin spin (T.sub.2) relaxation constants believed to be of medical diagnostic value in determining the state of health of tissue in the region examined. In the course of an examination, the patient region of interest is positioned in a substantialy uniform polarizing magnetic field produced by one of several known means, such as resistive, superconductive, or permanent magnets. Spectroscopic and imaging data are collected by subjecting the region of interest to pulse sequences comprised of magnetic field gradients and radio-frequency (RF) pulses. The magnetic gradient and RF fields are generated by separate coil assemblies positioned in the polarizing magnetic field and have generally cylindrical configurations to accommodate the patient region to be studied. The resonant frequency of the RF coil is selected based on the strength of the polarizing magnetic field and the type of nucleus (e.g., hydrogen, phosphorus, etc.) to be examined.
One of the effects of positioning a patient or another electrically conductive study object within the RF coil is that the coil is "loaded." The RF load is related to the quality factor Q, the coil resonant frequency, and impedance of the coil when the object is placed inside. Thus, for example, an "unloaded" RF coil may have a Q of approximately 350, while a coil with a 75 kg person positiond therein may have its Q lowered to 65. The load to the coil determines the amount of power required from the RF power amplifier necessary to perform the NMR experiment, and determines the level of noise which is included in the received NMR signal used to construct an image, for example. If the load on the RF coil is too low, the RF system will not be stressed adequately, and the noise in the image will not be representative of that found in an anatomical image.
Therefore, a phantom designed to simulate a patient must load the RF coil in a manner similar to an average person in order to provide valid test measurements. Additionally, the phantom must have several other important properties: The phantom or parts thereof must be sufficiently light to be easily carried from place to place; the phantom should allow the evaluation of the inherent polarizing magnetic field and radio-frequency magnetic field uniformity of the scanner apparatus and not itself be a source of this type of distortion in the imaging volume; the image produced by the phantom must include "noise" from all sources of incoherent noise common to clinical imaging (that is, RF coil load, NMR experiment parameters, polarizing and radio-frequency field non-uniformities), while not introducing any image artifacts, itself; the signal-to-noise ratio measurement must be derivable from an image of an object generating the NMR signal; the spin lattice and spin spin relaxation times, and spin density of the phantom material that generates the NMR signal used to produce an image must be representative of the NMR properties of human tissues commonly scanned; the design of the phantom must be such that voxel volume is determined by slice selection (slice profile) and field-of-view performance of the scanner system; and the phantom construction and geometry must be such that it can be scanned in any of the three orthogonal planes (transverse, sagittal and coronal) and present identical images for signal-to-noise evaluation in each of the planes.
Earlier phantom designs have attempted to make a single component phantom that provided both proper loading to the RF coil and generated an NMR signal for the evaluation of signal-to-noise and other performance parameters. The resulting device represented a substantial compromise in several of the requirements stated above.
Accordingly, it is a principal object of the present invention to provide an NMR phantom which satisfies, without significant compromise, the aforementioned phantom properties.